I think that turning megatron into a tank is a good idea, especially for the live action film. Im wondering how many times he has been a tank now. Including the upcoming lines, this is what I can think of..

Well, let's not speculate as to the movie Megatron. Rumor is that the tank is someone else and that Megs is a jet. Plus the Botcon BW Megs isn't a tank. It's an armored missle vehicle. Tanks have treads, he has wheels.

And the big green tank was the first G2 tank. The smaller Hero one that was also a robot master was the second one. Here's the list.

Considering 3 of those are pretty much the same incarnation, just different toys, he hasn't been a tank all that many times. Both G2s and the classics are the same desing, just different sizes. The new one will have more articulation, but same transformation.

Ultra Megatron (Battlestars) - he only ever showed up in the TV Magazine spreads, but I think he was some sort of tank. I know Super was a flying cannon, but Ultra in addition to the bulked up robot mode I believe had a new Alt mode.
Action Master Megatron - didn't transform, but he drove about in a tank. Given the other Action Masters who had vehicles had them largely inspired by their old alt modes (Prime - truck trailer. Wheeljack - racer. Starscream & Thundercracker - jets), then you can add this guy as well, though conversely, a gun couldn't exactly convert into a vehicle like a tank could.
G2 (Big)
G2 (Hero)
G2 (Hero repaint) - never released, but worthy of a mention.
G2 Dreadwing repaint - Dreadwing is primarily a bomber, however, he has a third mode that is a tank. If you're willing to count the Botcon repaint as a tank, you might as well count this as well.
Armada Megatron
War Within Megatron
Titanium Megatron
IDW Megatron (Stormbringer and Infiltration are the same Megatron, Simon confirmed he's a Cybertronian war machine of sorts, and that in Escalation he'll have a more Earthbound alt mode).
Botcon Megatron may not have treads, but in my mind he's close enough to a tank to be counted, especially given how broadly I'm reaching out for Alt modes here!
Robot Masters Megatron is remolded enough to count on his own, I feel - different head, much changed cannon, etc.

Then we've got the two pack version, who is actually more of an all new design. His following the same basic transformation scheme makes him as much like the G2 version as Energon Starscream is like Smokejumper. Totally new toy with a passing resemblance to the old.

Totalling it up: 13
Excluding media-only representations: 10
Throwing out the two who never came out, Botcon and AM Megatron: 6

Those six: G2 Tank, G2 Hero, Robot Masters, Titanium, Armada and the upcoming two pack release. That does make it the most common alt mode choice for Megatron, with a flying machine in second place, I think.

Though in terms of sheer badassitude, nothing beats his batmobile mode.

well, Im really hoping that Alternator/MP Abrhams tank color design sheet was real. That looks lke it could be the ultimate Megatron toy in my opinion.

Megs has been a:
tank 13 times
cannon twice (Galvatron and "Hearts of steel" Megs)
reptile about 6 0r 7 times
gun once
And in Beast Machines wasnt he some kind of Computer thingy?

Click to expand...

With the release of Classics Megatron he will have been a gun three time (G1 Megatron, G1 Galvatron's gun mode, and of course Classic). He has been a head at least two times, a hand twice, and a car at least seven times (RID Megatron and Galvatron, G2, and Cybertron Megatron and Galvatron counting leader and legends classes). Other forms are boat, dragon, alligator, bat, elephant, jet and space ship, t-rex, and basically Unicron in Energon. I'm probable forgetting a lot. Tank seems most common in comics and toys combined though.

Prime was the bat, Megatron was the crocodile. And the RiD Megatron/ Galvatron molds could turn into the elephant and hand (even though they were 'official' alt modes you still had to use your imagination though)

And Megatron's spark inhabited the giant floating head fortress in Beast Machines

In the series he was always portrayed as an immobile cannon in vehicle mode - the treads merely serving as a means of moving the cannon from one place to another, by towing it.
The only time you could even really argue that Galvatron's alt mode is a tank is in the PS2 G1 game, since in that he goes to cannon mode and zooms around on his treads. But then in that game, Megatron's gun mode was a floating cannon that needed nobody to fire it so take it as you will.